1. Field of the Invention
The invention concerns the use of eddy currents for testing a metal plate, in particular a metal plate with borings.
It is especially intended to be applied to testing of a steam generator tube sheet.
2. Prior Art
Eddy currents have already been used for testing steam generator tubes. These are tested on site with internal probes by means of eddy currents, at the time of the initial complete inspection of boilers and on subsequent inspections in service. The test is entirely automated, which minimizes the exposure of personnel to ionizing radiation. A robot allows positioning of the probe at the inlet of each tube in the tube sheet, introduction of the probe in the tubes, extraction of the probe at a constant speed and obtaining of data on graphic recorder charts or magnetic tape. To introduce the probe in the tubes, a selective positioning apparatus is used, such as that described in French Patent Application No. 74-39771 in the name of applicant.
Though testing of steam generator tubes is possible with a certain number of known methods, in particular by use of eddy currents, a method of testing a region located beyond the tubes, namely the tube sheet, has not existed to date.
In addition, the methods used to detect faults in tubes have been mostly directed towards detecting longitudinal fissures, extending along the axis of the tube. They do not detect transverse fissures.